Delta-9-tetrahydrocannabinol (THC) has been hypothesized to influence immune function. However, this has not been investigated in a comprehensive fashion in humans. The purpose of this study is to measure and study the effects of THC on immune function. To investigate immune-endocrine correlations, hormone parameters defining the activity of the hypothalamo-pituitary-adrenal axis have been measured during THC administration. (The effects of THC on cognitive function will also be investigated.) Experienced THC users have been recruited for study. Immune function of lymphocytes in vitro has been investigated during orally administered and inhaled THC and during the washout phase.